Always make a back up .
This won't wipe when you use home csc , this will put tmobile firmware , so use home csc in the csc slot .
This is the steps :
Windows computer needed
Latest Odin can Google it and download should be 3.13.1 or 3.14.1 / or Frija tool
Download samfirm v0.4.1 tool and frija, extract it with 7zip.
You will extract files for Odin 3.14.1once download with 7zip and will do same for firmware with 7zip .
Put your model number SM-G988U...region TMB select check for updates , then download the FIRMWARE , extract the FIRMWARE with 7zip.
There will be 4 files after you extract the firmware
BL
AP
CP
CSC
Each of those files will go in Odin when you open to the corresponding slots
BL
AP
CP
CSC USE HOME CSC FIILE TO SAVE DATA
Home data slot dont put nothing
connect your phone with cable to computer , once the files are in its slots hit start, once done you'll see pass in Odin and it will reboot
*Make sure you put phone in download mode
*If dont see your phone connected in Odin change cables or ports in computer .